The present invention relates to transparent conductive films using plastic films and to touch panels using the same. In particular, these films exhibit excellent durability to input with a pen when used in touch panels.
Transparent conductive films having transparent and low-resistive compound thin films formed on plastic films are widely used for various uses utilizing their conductivity, for example, flat panel displays such as liquid crystal displays and EL displays, transparent electrodes of touch panels, and other uses in the electrical and electronic field.
For transparent conductive thin films, typical examples of those for general use are tin oxide, indium oxide, indium-tin composite oxide, and zinc oxide. For their substrates, various kinds of plastic films have been used, including polyethylene terephthalate.
In recent years, with the diffusion of portable information terminal devices, there has been raised a demand for touch panels having the function to input character with a pen. The transparent conductive films used in the touch panels for input with a pen are required to have the following:
(1) transparent conductive thin films are in strong adhesion to silver paste; and
(2) no occurrence of cracks or peeling will be caused in the transparent conductive thin films of the transparent conductive films by pressurization with a pen.
To these requirements, conventional transparent conductive films have the problems as described below.
For the purpose of achieving strong adhesion between silver paste and transparent conductive thin films, baking after the application of silver paste onto the transparent conductive thin films may be carried out at 150xc2x0 C. or higher temperatures. However, if the ordinary polyethylene terephthalate films are heat treated at 150xc2x0 C., the films will become cloudy in white by the deposition of oligomers. Therefore, a transparent conductive film has been proposed (JP-A 60-131711), in which a layer produced by the hydrolysis of an organosilicon compound is provided as an oligomer deposition preventing layer on a transparent plastic film and a transparent conductive thin film is further laminated thereon. However, this transparent conductive film requires heat treatment at 150xc2x0 C. for 10 hours for cross-linking the layer, which has been produced by the hydrolysis of an organosilicon compound, after the film formation of the transparent conductive thin film. For this reason, it results in a crystalline transparent conductive thin film, whereby the etching characteristics of the transparent conductive thin film at the time of touch panel production will be extremely deteriorated and the cost of touch panel production will become high.
In addition, when transparent conductive films are used in the touch panel for input with a pen, a pair of conductive thin films, which are disposed opposite to each other with a spacer interposed therebetween, come together into strong contact with each other by pressurization for input with a pen, so that the occurrence of cracks or peeling will be caused in the thin films, thus leading to some troubles including an increase in electrical resistance and an occurrence of disconnection.
Therefore, a transparent conductive film has been proposed (JP-A 2-66809), in which a transparent conductive thin film is formed on a transparent plastic having a thickness of 120 xcexcm or smaller and its lamination with another transparent substrate is achieved with an adhesive layer. The film, however, has insufficient durability to input with a pen. Further, because of lamination with an adhesive, foreign matter such as dust will be incorporated at the time of lamination, resulting in a transparent conductive film with many optical defects.
There has also been proposed a transparent conductive film (JP Patent No. 2525475), in which the crystalline particle size of indium oxide is made to 0.3 xcexcm or smaller by heating treatment. However, this transparent conductive film becomes a crystalline transparent conductive thin film. For this reason, the etching characteristics of the transparent conductive thin film at the time of touch panel production will be extremely deteriorated and the cost of touch panel production will become high, as described above.
There can further be mentioned, for example, a transparent conductive film (JP-A 64-9729) having a structure in which an aqueous polyester resin is provided between a base film and a conductive later; and a transparent conductive film JP-A 2-279106, in which a thermoplastic polyester-type resin is provided between a base film and a conductive layer.
However, these aqueous polyester resin and thermoplastic polyester-type resin have poor heat resistance because they have no cross-linked structure, and therefore, the occurrence of minute wrinkles will be caused by heat when conductive paste or other materials are baked on the transparent conductive film, thus leading to a deterioration of appearance.
The present invention is directed to the solution of the above conventional troubles, and the first objective thereof is to provide a transparent conductive film with an improvement in durability to input with a pen when used in touch panels, which is achieved by an improvement in the adhesion of a conductive thin film. The second objective thereof is to provide a transparent conductive film in which a transparent conductive thin film can be made into strong adhesion to silver paste when used in touch panels.